1. Field of the Invention
The invention relates to the field of power electronics. It relates to a protective circuit with feedback capability for two-point inverters or three-point inverters. The invention furthermore relates to a method of feeding back the energy stored in the protective circuit.
2. Discussion of Background
In order that the semiconductor switches of high-power inverters are not destroyed by excessively high current rise (di/dt) or voltage rise (du/dt), they have to be protected by a protective circuit ("snubber"). In addition, the protective circuit must be designed so that a short turn-on/turn-off duration is achieved for the semiconductor switches and the semiconductor switches are stressed by as little peak voltage as possible during turn-off. A large number of circuit proposals already exist for such protective circuits.
It is particularly advantageous if the protective circuits operate in as loss-free a manner as possible. This is best achieved if the energy stored in the protective-circuit capacitors and protective-circuit coils can be fed back again into the direct-current source. Such a protective circuit with feedback capability for two-point inverters has already been disclosed in DE 39 37 111; a protective circuit with feedback capability for three-point inverters has already been disclosed in DE 38 33 700.
In these two publications, the energy stored in the protective circuit is fed back via a DC/DC converter into the direct-current source, i.e. the so-called link capacitor. As a result, the circuit becomes active and has low loss. In this case, the DC/DC converter preferably comprises a transformer with two primary windings and a secondary winding. The unidirectional connections of the primary winding are each connected via a feedback diode to one input of the DC/DC converter in each case. A feedback capacitor and a resistor are connected in parallel at the input of the DC/DC converter. The secondary winding of the three-winding transformer is connected to the link-circuit voltage via a full-wave rectifier comprising four diodes. The power loss produced, which is capable of feedback, is dissipated in the resistor connected in parallel at the input of the DC/DC converter.
A disadvantage of this circuit is that the resistor limits the efficiency of the circuit. Equally, excessively large values have to be accepted for the turn-on/turn-off duration. Although a larger resistance results, on the one hand, in a better efficiency, the energy can no longer, on the other hand, be removed so rapidly because of the increased time constant; specifically, the semiconductor switches can be switched again only after the protective-circuit energy has been removed. An increasing amount of energy will otherwise be stored in the protective circuit and there would be the risk of destruction. A longer turn-on/turn-off duration of the semiconductor switches has therefore to be accepted.